Vapor compression systems, such as heat pumps, refrigeration and air-conditioning systems, are widely used in industrial and residential applications. The introduction of variable speed compressors, variable position valves, and variable speed fans to the vapor compression cycle has greatly improved the flexibility of the operation of such systems. It is possible to use these new components to improve the efficiency of vapor compression systems by controlling the components correctly.
For example, a speed of the compressor can be adjusted to modulate a flow rate of a refrigerant. The speed of an evaporator fan and a condenser fan can be varied to alter heat transfer coefficients between air and heat exchangers. The change in an expansion valve opening can directly influence a pressure drop between the high-pressure side and the low-pressure side of the vapor compression system, which, in turn, affects the flow rate of the refrigerant as well as superheat at the corresponding evaporator outlet.
The combination of commanded inputs to the vapor compression system that delivers a particular amount of heat is often not unique and these various combinations consume different amounts of energy. Therefore, it is desirable to operate the vapor compression system using the combination of inputs that minimizes energy and thereby maximizes efficiency.
Conventionally, methods maximizing the energy efficiency rely on the use of mathematical models of the physics of vapor compression systems. Those model-based methods attempt to describe the influence of commanded inputs of the components of the vapor compression system on the thermodynamic behavior of the system and the consumed energy. In those methods, models are used to predict the combination of inputs that both meets the heat load requirements and minimizes energy.
However, the use of mathematical models for the selection of optimizing inputs has several important drawbacks. Firstly, models rely on simplifying assumptions in order to produce a mathematically tractable representation. These assumptions often ignore important effects or do not consider installation-specific characteristics such as room size, causing the model of the system to deviate from actual behavior of the system.
Secondly, the variation in these systems during the manufacturing process are often so large as to produce vapor compression systems of the same type that exhibit different input-output characteristics, and therefore a single model cannot accurately describe the variations among copies produced as the outcome of a manufacturing process.
Thirdly, these models are expensive to derive and calibrate. For example, parameters that describe the operation of a component of a vapor compression system, e.g., a compressor, are experimentally determined for each type of the compressor used, and a model of a complete vapor compression system may have dozens of such parameters. Determining the values of these parameters for each model is an extensive effort. Finally, vapor compression systems are known to vary over time. A model that accurately describes the operation of a vapor compression system at one point in time may not be accurate at a later time as the system changes, for example, due to slowly leaking refrigerant or the accumulation of corrosion on the heat exchangers.
FIG. 1 shows a conventional vapor compression system 100 that includes components, e.g., variable setting actuators. The components may include an evaporator fan 114, a condenser fan 113, an expansion valve 111 and a compressor 112. The system can be controlled by a controller 120 responsible for accepting setpoints 115, e.g., from a thermostat, and readings of a sensor 130, and outputting a set of control signals for controlling operation of the components. The controller 120 is operatively connected to a set of control devices for transforming the set of control signals into a set of specific control inputs for corresponding components. For example, controller is connected to a compressor control device 122, to an expansion valve control device 121, to an evaporator fan control device 124, and to a condenser fan control device 123.
In this manner, the controller controls operation of the vapor compression system such that the setpoint values are achieved for a given heat load.
However, the operation of the system can be not optimal. In consideration of the above, there is a need in the art for a method for controlling operation of the vapor compression system such that heat load of the operation is met and a performance of the system is optimized, where the method is not model-based and can adapt over time as the system characteristics evolve.